Linux Unwired [Electronic resources] نسخه متنی

7.2 Bluetooth Basics

Bluetooth Special Interest Group (SIG), a
consortium of telecommunications, electronics, and computer
manufacturers, develops Bluetooth. The founding members were
Ericsson, Nokia, IBM, Intel, and Toshiba. The first version of the
Bluetooth
specification was formally adopted by the SIG in 1999.

The first revisions of the Bluetooth specification had a mixed
reception, because implementations were dogged by interoperability
problems. The 1.1 release, published in 2001, eliminated the gray
areas from the 1.0b specification and, as a result, improved device
interoperability. Over two years since the 1.1 release, Bluetooth is
well on its way to becoming a ubiquitous technology in portable
devices. At the time of writing, the current approved revision of the
Bluetooth specification is Version 1.2, released in November 2003.

The Bluetooth specification itself covers the many levels involved in
getting a signal between two applications, from the radio through
link control to application-level protocols. Figure 7-1 shows just some of the various strata
specified by Bluetooth, which we encounter in this chapter. Further
details, including the specifications themselves, can be obtained
from http://www.bluetooth.org.

Figure 7-1. Some layers of the Bluetooth specification

Bluetooth hardware typically takes the form of one or two microchips,
which are embedded in devices. Computers are increasingly shipping
with integrated Bluetooth adapters, but the prevailing way of adding
Bluetooth support is by adding an external adapter, typically via the
USB or PC card ports. Before a device can sport the Bluetooth logo
and use the Bluetooth trademarks, it must be put through a series of
tests known as qualification. Qualification
involves tests for all parts of the Bluetooth specification, from
radio testing to protocol conformance.

7.2.1 What You Can Do with Bluetooth

As Bluetooth is intended to replace cable,
it can be used for more or less the same purposes as a cable, within
the bandwidth constraints of the technology. All the following usage
scenarios are supported within Linux and are discussed in this
chapter:

Serial port

Bluetooth's serial port emulation
enables the connection of modems (such as in cell phones) and
printers. Serial emulation is also an easy way of writing simple data
exchange applications using Bluetooth.

Object exchange (OBEX)

Facilitated by implementing the OBEX protocol,
object exchange is used for
"beaming" data objects between
devices, such as contacts from address books. It is the main way that
cell phones exchange data and is often implemented by
Bluetooth-enabled printers.

Synchronization

Devices that implement the IrMC specification permit
synchronization
of data sources, such as calendars and address books. Many
Bluetooth-enabled cell phones have this feature. More modern devices
implement the SyncML specification, which requires a networking
connection.

Networking

Bluetooth supports two different forms of
networking. The most basic and
commonly implemented form is dial-up networking using PPP over a
serial connection. In addition, there is BNEP, an encapsulation of
Ethernet networking, which allows Bluetooth devices to join a network
in a manner much more analogous to Wi-Fi networking.

Input devices

Bluetooth supports an array of input devices similar to USB. Major
manufacturers such as Apple and Microsoft are shipping
Bluetooth-enabled mice and keyboards.

Audio

Audio is one of the most-promoted aspects of Bluetooth by cell phone
manufacturers; it is possible to support bi-directional
audio
connections to headsets over Bluetooth.

7.2.2 Concepts

The following sections describe essential Bluetooth concepts that you
need to be aware of. These include the Bluetooth address, which
uniquely identifies a Bluetooth adapter; the protocols and profiles
that define the communication techniques and device capabilities;
bonding, discoverability, and device classes, which Bluetooth devices
use to find each other and communicate their abilities; and piconets,
scatternets, masters, and slaves, which describe the topology of
Bluetooth networks and the relationships of one device to another.

7.2.2.1 Bluetooth address

Each Bluetooth interface has a Bluetooth
address,
also known as its BDADDR. These addresses look
very much like Ethernet interface MAC addresses, and follow the same
address allotments that the ANSI/IEEE 802 standard, administered by
the IEEE, has laid down. The first three octets of the Bluetooth
address denote the organizationally unique identifier (OUI). For
instance, the address 00:80:98:23:15:6E has an OUI
of 008098, which is registered to the TDK Corporation.

OUIs can be looked up online using the IEEE's search interface at http://standards.ieee.org/regauth/oui/. As some device manufacturers subcontract to others, it may not always be possible to determine the manufacturer of a device from its OUI.

In addition,
Bluetooth
adapters have a programmable name used to present to the user in
interactions. Example 7-2 shows both the Bluetooth
addresses and the names that are discovered in a device scan.

7.2.2.2 Protocols

The
Bluetooth
specification defines some protocols of its own and also reuses some
existing standards. A protocol is an agreement about the way data is
exchanged. It is on top of these protocols that all applications of
Bluetooth are built. An in-depth knowledge of the protocols is not
necessary to deploy Bluetooth, but passing familiarity with them
helps in troubleshooting situations.

Confusingly, some of the protocols have very similar names to the
profiles in which they are used and are listed
next. (Additionally, some protocols are layered on top of lower-level
protocols. This happens elsewhere in computingfor example,
when a computer connects to the Internet via a modem, it uses the
RS232 protocol to communicate serial port data, the PPP protocol on
top of that to facilitate a network connection, and TCP/IP on top of
that to carry the data.)

Link Manager Protocol (LMP)

Provides basic control of interdevice communication links

Logical Link Control and Adaptation Protocol (L2CAP)

Provides logical channels of communication to higher protocol layers

Radio Frequency Communication (RFCOMM)

Provides emulated serial connections

Object Exchange (OBEX)

A simple file transfer protocol

Bluetooth Network Encapsulation Protocol (BNEP)

Provides Ethernet encapsulation for wireless networking

Service Discovery Protocol (SDP)

Enables the querying and reporting of services that a device supports

Telephony Control Protocol Specification (TCS)

Provides call control for voice and data telephone calls

7.2.2.3 Profiles

A profile is the name given to the
implementation of one more protocols to provide a particular
application service.
Bluetooth
devices advertise profiles. Many of the profiles build on each
otherfor instance, the OBEX profile builds on the serial port
profile.

Commonly implemented profiles include:

Service Discovery Access Profile (SDAP)

Enables a device to discover the profiles supported by other devices

Serial Port Profile (SPP)

Emulates a serial port connection

Hardcopy Cable Replacement (HCRP)

Emulates a parallel port connection for the purposes of printing

Dial-up Networking Profile (DUN)

A connection to a modem or cell phone, which connects to an Internet
access point

LAN Access Profile (LAP)

A point-to-point (PPP) access to a network

Headset Profile (HS)

A combination voice and control channel, which provides a link
between a cell phone and audio headset

Generic Object Exchange Profile (GOEP)

A file exchange, which exchanges business cards on cell phones

File Transfer Profile (FTP)

Analogous to Internet FTP, which allows navigation and access to a
filesystem

Synchronization Profile (SP)

An address book and calendar synchronization, which uses the IrMC
protocol

Human Interface Device Profile (HID)

A connection to a keyboard, mouse, joystick, barcode scanner, or
other input devices

Personal Area Networking (PAN)

An Ethernet-like access to a network

Basic Printing Profile (BPP)

Enables devices to print text, as well as formatted documents; useful
for low-powered devices such as phones or pagers

7.2.2.4 Bonding

Bonding, also called pairing, is the process by which trust is
established between two
Bluetooth
devices. The user is required to input matching codes, called
personal identification numbers (PINs), into the
two devices. In some situations, one of the devices may have the PIN
pre-setfor example, some headsets come with a
PIN
of 0000. PINs are typically a sequence of digits; they provide little
security, and they are intended only for the initial pairing.

Given a successful match of PIN, the devices negotiate a
link key, a much more cryptographically secure
code, which is used thereafter as an access control mechanism between
the two devices.

7.2.2.5 Discoverability

A
Bluetooth
device is discoverable if it can be found by
another device's inquiry. During discovery, the
inquiring device broadcasts a specially coded message. As remote
devices receive the message, they send a return message indicating
their presence. In most circumstances, you must make a device
discoverable in order to initiate bonding.

7.2.2.6 Device classes

Bluetooth devices fulfill many
functions, so there should be a way that a device can quickly
indicate its primary function. As we have already mentioned, the SDP
exists to provide a complete description of running services.
However, Bluetooth provides an additional way for a device to
describe itself: the device class. Although the
SDP provides the description of the running services, the device
class provides the purpose of them.

The device class code is a 24-bit number that incorporates three
subcodes: the major device type, the minor device type, and
additional service codes, which broadly indicate the services
available. Table 7-1 shows the meaning of the
useful major device types (other types are reserved or undefined),
and Table 7-2 shows the useful service class bits.
The meaning of the minor device type bits (bits 7-0) depends on the
major device type. You can find a full explanation of these values on
the Bluetooth Special Interest Group web site at http://www.bluetoothsig.org/assigned-numbers/baseband.

0 0 0 0 0

0 0 0 0 1

0 0 0 1 0

0 0 0 1 1

0 0 1 0 0

0 0 1 0 1

0 0 1 1 0

1 1 1 1 1

16

17

18

19

20

21

22

23

7.2.2.7 Piconets and scatternets, and masters and slaves

A
piconet
is a network of Bluetooth devices created by a master connecting to
one or more slaves. The master is the device that initiates the
connection. Figure 7-2 shows the topology of a
piconet. A master may be connected to as many as seven slaves
simultaneously.

Figure 7-2. Topology of a piconet

Various applications such as LAN access points require the
master/slave
relationship to be the same as the server/client relationship. For
this reason, a client device, which serves as a master, initiates a
connection to the access point; once connected, a
role-switch occurs, and the client device now
becomes a slave. For most applications of Bluetooth on Linux, you do
not need to be aware of these distinctions, but the knowledge of
their existence may be useful in debugging scenarios. Some Bluetooth
hardware has restricted role-switching ability.

Sometimes, a slave in one piconet is connected to a master of another
piconet. The linking together of multiple piconets in this way is
called a
scatternet.